What is mean by 3 level program memory lock?

Question

I am building my first microcontroller programmer using at89c51. In its documentation I have read that it support 3 level programming lock. But I did not find a simple explanation of this term.

Will somebody explain what are memory locks and what is mean by 3 level in this context?

Answer 1

Its purpose seems to be to restrict access to the program memory, to prevent reading out the program from a controller.

There are three levels (plus one unprotected level), see the bottom of page 5.